Higher operating temperatures for gas turbine engines and the like are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high-temperature durability of the components of the turbine must correspondingly increase. Significant advances in high-temperature capabilities have been achieved through the formulation of nickel, cobalt and iron-based superalloys. These superalloys may be designed to withstand temperatures in the range of about 1000 to 1100 degrees C. or higher. Nonetheless, when used to form components of the turbine, such as combustor liners, augmentor hardware, shrouds and high and low-pressure nozzles and blades, the superalloys are often susceptible to damage by oxidation and hot corrosion attack and may not retain adequate mechanical properties. For this reason, these components are typically protected by an environmental and/or thermal-insulating coating, the latter of which is referred to as a thermal barrier coating (TBC). Ceramic materials, and particularly yttria-stabilized zirconia (zirconium oxide) (YSZ), are widely used as TBCs. These materials are employed because they may be readily deposited by plasma-spraying, flame-spraying and physical vapor deposition (PVD) techniques, and because they generally exhibit desirable thermal characteristics. In general, these TBCs may be utilized in conjunction with the superalloys in order to reduce the cooling air requirements associated with a given turbine.
In plasma-spraying, an electric arc is typically used to heat various gasses, such as air, nitrogen or hydrogen, to a temperature of about 8000 degrees C. or higher. These gasses are expelled from an annulus at high velocity, creating a characteristic flame. Powder material is fed into the flame and the melted particles are accelerated toward the substrate being coated. In PVD, an ingot of a ceramic material being deposited on the substrate is placed in an evacuated chamber. The top end of the ingot is heated by an intense heat source, such as an electron beam or laser, such that it melts and forms a pool. A portion of the hot, molten ceramic evaporates and condenses on the substrate, and a coating is gradually formed as the ingot is raised to replenish the pool. The coatings resulting from plasma-spraying and PVD are typically of good quality and durability. However, localized spallation of a TBC may occasionally occur during the manufacturing process, regardless of the technique utilized, due to surface contamination, handling damage or the like.
In order to be effective, TBCs must have low thermal conductivity, must strongly adhere to the component and must remain adhered through many heating and cooling cycles. The latter requirement is particularly demanding due to the different coefficients of thermal expansion between the ceramic materials and the superalloy substrates that they protect. To promote adhesion and extend the service life of a TBC, an oxidation-resistant bond coating is often employed. A bond coating typically takes the form of an overlay coating, such as MCrAlX (where M is iron, nickel and/or cobalt and X is yttrium or another rare earth element), or a diffusion aluminide coating, such as nickel aluminide or platinum-modified nickel aluminide. During the deposition of the TBC and subsequent exposures to high temperatures, such as during turbine operation, these bond coatings form a tightly adherent alumina (aluminum oxide) (Al2O3) layer or scale that adheres the TBC to the bond coating.
The service life of a TBC is typically limited by a spallation event brought on by thermal fatigue, contaminants present during the coating process, contact during turbine manufacture/assembly/operation, erosion, metallurgical issues, etc. Accordingly, a significant challenge has been to obtain a more adherent ceramic layer that is less susceptible to spalling when subjected to thermal cycling and the like. Though significant advances have been made, there is the inevitable requirement to repair components whose TBCs have spalled. Though spallation typically occurs in localized regions or patches, the conventional repair method has been to completely remove the TBC (after removing the affected component from the turbine or the like), restore or repair the bond coating as necessary and recoat the entire component, especially when plasma-spraying or PVD techniques which do not work well in constricted areas are utilized. Techniques for removing TBCs include grit-blasting and chemical stripping with an alkaline or caustic solution at high temperatures and pressures. Grit-blasting, however, is a time-consuming, labor-intensive, expensive process and often erodes the surface beneath the coating. With repetitive use, grit-blasting may eventually destroy the component. The use of an alkaline or caustic solution to remove a TBC is also less than ideal, since the process requires the use of an autoclave operating at high temperatures and pressures. Consequently, conventional repair methods are time-consuming, labor-intensive and expensive, and may be difficult to perform on components with complex geometries, such as airfoils and shrouds.
As an alternative to the processes described above, U.S. Pat. No. 5,723,078 to Nagaraj et al. discloses selectively repairing a spalled region of a TBC by texturing the exposed surface of the bond coating and depositing a ceramic material on the textured surface by plasma-spraying. While avoiding the necessity of stripping the entire TBC from a component, the repair method requires the removal of the component from the turbine in order to deposit the ceramic material. Thus, masking and over-spraying problems arise and time, effort and money are wasted.
In the case of large power-generation turbines, completely halting power generation for an extended period of time in order to remove components whose TBCs have suffered only localized spallation is not economically desirable. As a result, components identified as having a spalled TBC are often analyzed to determine whether the spallation has occurred in a high-stress area. A judgment is then made as to the risk of damage to the turbine due to the reduced thermal protection of the component, which if excessive may lead to catastrophic failure of the component. If the decision is made to continue operation, the spalled component must typically be scrapped at the end of operation because of the thermal damage inflicted while operating the component without complete TBC coverage.
As another alternative to the processes described above, U.S. Pat. Nos. 5,759,932 and 5,985,368 to Sangeeta et al. disclose a TBC patch formed from a slurry composition including hollow spheres of zirconia contained within a porous oxide matrix derived from silica, such as aluminosilicate. The preferred matrix material is mullite (3Al2O32SiO2). The TBC patch method includes applying a slurry base coating having about 0 to 40% zirconia spheres by weight and one or more slurry top coatings each having about 25 to 99% zirconia spheres by weight. Various heat treatments are applied to the base coating and top coatings in order to cure them. While providing a TBC patch with a relatively high thermal expansion coefficient and thermal stability (melting point), along with a relatively low thermal conductivity, this method provides relatively low erosion resistance due to the use of the hollow spheres of zirconia.
As a further alternative to the processes described above, U.S. Pat. No. 6,413,578 to Stowell et al. discloses selectively repairing a spalled region of a TBC by cleaning the surface of the component exposed by the localized spallation and applying a ceramic paste including a ceramic powder in a binder to the surface, the ceramic powder including alumina and zirconia, the binder being a ceramic precursor binder that thermally decomposes to form a refractory material. The binder is heated to yield a repair coating that covers the surface, the repair coating including the ceramic powder in a matrix of the refractory material formed by reacting the binder with the alumina, i.e., a network of mullite.
What is still needed, however, is an in-situ repair method that may be performed on localized spalled areas of the TBC of a component of a turbine or the like without necessitating that the component be removed from the turbine, minimizing downtime and scrappage. The TBC patch must have a high thermal expansion coefficient, high thermal stability (a high melting point), low thermal conductivity, good adherence properties and high erosion resistance. In addition, it is desirable to have a TBC patch with a composition that is similar to the composition of the surrounding, unspalled TBC, in order to closely match the properties of the unspalled TBC. Thus, what is needed is a repair method that does not utilize hollow spheres of zirconia and that does not necessitate the formation of a network of a second ceramic phase through the reaction of silica with additional ceramic powders including alumina. The TBC patch should be able to be cured at a wide range of temperatures and under a wide range of conditions and the zirconia used should remain as a solid.